Plastics are widely used in both durable and non-durable applications. The majority of non-durable plastics are used in initial packaging of goods and food service packaging, often in single use applications.
Using solid-state foaming for producing layered cellular structures in the context of a batch process may take hours to diffuse gas into the sheet, and the gas begins to diffuse from the sheet upon removal from a pressure vessel. The uneven amount of time in processing a roll after removal from a pressure vessel may result in non-uniform foam properties. The long processing times and resulting non-uniformity in rolls may require a large number of large, expensive pressure vessels in order to process plastic continuously.
For example, U.S. Pat. No. 8,591,799 teaches that 2 hours to 60 days are needed for saturating the solid with the inert gas. (col 5, lines 10-27). Thermoplastic polymers disclosed include polycarbonate, polypropylene, polyethylene, polyethylene terephthalate, polyvinyl chloride, poly(lactic acid), acrylonitrile butadiene styrene, and poly styrene. (col 4, lines 48-53) Pressures of 3-7.5 MPa are taught by that patent. (col 2, lines 23-27).
U.S. Pat. No. 8,377,548 utilizes 72 hours in Example 1 and 36 hours in Example 2 for the inert gas diffusion step. Disclosed thermoplastic materials included PET, PEEK, PEN, PBT, PLA, PHA, and thermoplastic urethane. (abstract). Examples 1 and 2 utilized PET polymer, CO2 gas and a pressure of 5 MPa.